The present embodiments relate to cascaded multi-level inverters. In particular, the embodiments relate to fault-bypass operation of such inverters.
Cascaded multilevel inverters are used in industrial control systems. For example, cascaded multilevel inverters are used to control medium voltage motor drives (e.g., 4.1 kV-13.8 kV motor drives) and/or static voltage compensators.
Cascaded multilevel inverters are modular. The high power inverter is made-up of a number of smaller series connected single-phase inverters or cells. Any number (e.g., 3-8) of cells may be used for a given leg of a three-phase system. When one or more of these cells fail, the inverter can still operate and produce balanced line-line voltages by computing different voltage references. Although the fault-bypass maximum line-line voltage is less than in normal conditions, continued operation at the reduced power level may be preferable to a complete shut-down.
To provide balance, each leg may be operated with the same number of cells. Where a number of cells fail in one leg, the other legs may bypass a corresponding number of cells. However this approach does not provide maximum available voltage to the motor since not all available cells are used. In another approach, a feedback mechanism is used to generate the voltage references. However, the accuracy of the method is influenced by the feedback gain, which has to be fairly high for good results. This approach makes it more challenging when the output frequency of the cascaded multilevel inverter is high as the gain cannot be increased beyond certain limits.